DFT study on the Au(I)-catalyzed cyclization of indole-allenoate: Effects of counterion and solvent

Abstract

A computational study with the B3LYP density functional is carried out to explore the effects of counterion and solvent on the Au(I)-catalyzed cyclization reaction of indole-allenoate to form dihydrocyclopenta[b]indole derivatives. The optimal reaction path includes a intramolecular cyclization and a proton transfer. In the first process, the counterions Cl-, BF4- and OTf- can act as the hydrogen-bond acceptor to promote the intramolecular cyclization between C1 and C5 atoms. In the proton transfer, the anions can greatly reduce the energy barrier of proton migration as proton-transfer shuttle. More importantly, the Bronsted/Lewis basicity of counterions (Cl- > OTf- > BF4-) turns out to be the primary reason for the difference of counterion catalytic activity in the proton-transfer process. In the protonation of counterion, the catalytic capacities of counterions show significant difference according to the series Cl- > OTf- > BF4-, but the catalytic ability order of counterions is Cl- < OTf- < BF4- in the deprotonation of counterion-H. Iinterestingly, the strong coordinating capability of solvents (DMF and DMSO vs. PhCH3) is found to be another important factor that critically affects the reaction yield (entries 3-5, Table 1). In a word, our calculations not only explain the experimental phenomena well, but also put forward some guidance and advice for the selection of counterions and solvents in the transition metal-catalyzed reactions including the proton-transfer process.

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